The present invention relates to packaging for power and other circuitry, such as power semiconductor devices and associated circuitry.
Packaging for power circuitry must solve a number of mechanical and electrical problems. As examples of mechanical considerations, the package must provide for conducting heat away from power dissipating devices, for sealing the devices from the environment to protect the devices, and for reliably mounting the devices with respect to one another both mechanically and electrically. Electrically, the package must provide for high current carrying capability, high voltage isolation and low inductance connections. Moreover, it is desirable to integrate as much of the associated circuitry required to drive, control or protect the power semiconductor devices as possible in a single package, imposing further requirements on the packaging.
There have been a number of approaches taken to packaging power circuitry. Typically, a rigid substrate such as metal or ceramic is provided as a base. Where ceramic is employed, it may be of a type that is thermally conductive. Where metal is employed, an insulating layer is applied over the metal (known in the art as xe2x80x9cinsulated metal substratexe2x80x9d or xe2x80x9cIMSxe2x80x9d). As in standard printed circuit board construction, a layer of copper is plated over the ceramic or insulating layer and etched to form traces and pads for bonding and connecting power devices, such as bipolar junction transistors, field effect transistors, insulated gate bipolar junction transistors (xe2x80x9cIGBTsxe2x80x9d), diodes or silicon controlled rectifiers (xe2x80x9cSCRsxe2x80x9d). Power devices are soldered to the pads, and wires are bonded to the pads to make additional electrical connections.
Leads are provided to extend from the package and are wire bonded or soldered to the power devices. A xe2x80x9clead framexe2x80x9d may be used for this purpose, which is typically an assembly of nickel plated copper. The entire package is typically finally encapsulated in a plastic resin.
It is often desirable to provide a driver, control or protection circuit (hereinafter xe2x80x9ccontrol circuitxe2x80x9d) in a module of one or more power devices for driving the devices. This provides a higher level of integration, which leads to economies in production. Moreover, it is desirable to place the control for the power devices as close as possible to the power devices to minimize the inductance of the conductive lengths of wire or traces therebetween.
The control circuit is typically formed on a standard printed circuit board. To mount the printed circuit board to the base, header pins are oriented vertically and soldered to pads on the base to make physical connection thereto. Holes are provided in the printed circuit board to align with and receive the header pins as the board is placed over the base. The header pins are then soldered to the circuit board. Additional hardware, such as stand-offs and screws, are used to fix the mounting. The stand-offs and the header pins in combination provide mechanical support for the circuit board, while electrical connection between the circuit board and the power devices below is typically made through the header pins wherein additional connections may be made by wires.
A problem with this approach is that solder connections are both weak and brittle. The circuit board may break free from its mounting during assembly or during the encapsulation process, which risks breaking the electrical connection between the controller and the power devices. Moreover, during use, thermal expansion and contraction of the module stresses the joints and may ultimately cause failure.
Another problem with this approach is that the header pins are relatively large, so that the header pins and the holes required therefor in the circuit board consume valuable space on the circuit board, and contribute to increasing the size of the module as a whole.
Both the space required by the header pins and stand-offs and the brittleness of the solder connection of the header pins compound the difficulty of encapsulating the package. Transfer molding is typically used for packaging DIPs, discrete transistors, SOICs, and other smaller integrated circuits, because it has a number of advantages. It does not require that the device to be packaged have a separate enclosure to receive a resin material, it is fast, the high heat and pressure applied reduce the tendency to form voids and retain moisture in the package, and it provides a strong, rigid assembly that prevents the wires or leads inside the package from moving and, therefore, from being strained during power cycling and during use.
However, transfer molding has been limited to use in relatively small packages, primarily because of the increased propensity to form voids as the package becomes larger. Another problem with using transfer molding for larger packages is that the integrated circuits and wires in the package will in general reach a higher temperature and retain that higher temperature for a longer period of time during molding due to the reduced surface area of the package relative to its volume. Very high temperatures may stress the integrated circuits and melt the solder connections. Prior art power modules that include an auxiliary circuit board have not heretofore been small enough to take advantage of the advantages afforded by the transfer molding process.
Accordingly, there is a need for packaging for power and other circuitry that provides for interconnecting a circuit board and power devices mounted separately from the circuit board in an integrated power circuit more reliably and at lower cost, and that provides for minimizing the size of the package, and that provides for the use of transfer molding to encapsulate the package.
The packaging for power and other circuitry of the present invention solves the aforementioned problems and meets the aforementioned needs by providing a circuit board for mounting control circuitry and a power lead frame for mounting power circuitry. In addition to providing electrical connections to power circuitry, the lead frame is employed to fix or partially fix the location of the circuit board in the package. For this purpose, one of the power lead frame and the circuit board includes a male portion and the other of the power lead frame and the circuit board includes a complementary female portion for mechanically coupling the circuit board and the lead frame.
In a preferred embodiment of the invention, the lead legs include respective apertures for receiving corresponding tabs of the circuit board. The lead legs bend elastically to provide clearance for the tabs and elastically relax to capture the tabs in the apertures once these features mate.
In another aspect of the invention, a length of flexible material supporting electrically conductive traces is provided to electrically connect the printed circuit board to the power lead frame or to the power devices directly.
In yet another aspect of the invention, the flexible material is relieved on one or both sides to provide access to the traces for welding the traces to the lead frame.
Therefore, it is a principal object of the present invention to provide a novel and improved packaging for power and other circuitry.
It is another object of the present invention to provide packaging for power and other circuitry that provides for decreasing the cost of combining a printed circuit board and power devices in the same package.
It is yet another object of the present invention to provide packaging for power and other circuitry that provides for increasing the reliability of the electrical connection between circuitry mounted on one substrate and circuitry mounted on another, separate substrate.
It is still another object of the present invention to provide packaging for power and other circuitry that provides for increasing the reliability of the mechanical connection between a circuit board and another, separate substrate.
It is a further object of the present invention to provide packaging for power and other circuitry that provides for decreasing the manufacturing cost of providing a mechanical connection between a circuit board and another, separate substrate.
It is still a further object of the present invention to provide packaging for power and other circuitry that provides for flexibly mounting a circuit board to a rigid substrate.
It is yet a further object of the present invention to provide packaging for power and other circuitry that provides for more inexpensively electrically connecting a flexible connection to a substrate.
It is another object of the present invention to provide packaging for power and other circuitry that provides for decreasing the area required by the circuitry.
It is still another object of the present invention to provide packaging for power and other circuitry that provides for decreasing the overall size of the package.
It is yet another object of the present invention to provide packaging for power and other circuitry that provides for the use of transfer molding to encapsulate the package.
The foregoing and other objects, features and advantages of the present invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the following drawings.